Include all the necessary pieces to get the final Z position after
probing the bed, and add commentary about other possible changes.

Updates to `set_homing_bump_feedrate`:
- Move the string into Program Memory, reduce length by 31 bytes
- Use an auto to get the divisor, adjust it on error
- Set feedrate once, at the end

The compiler may be able to optimize if it knows an array won’t be
changing.

`gcode_M84` only needs to call `finishAndDisableSteppers`, which calls
all the other functions.

- Update prepare_move_delta to take a target argument
- Add Delta support to plan_arc

- Have `Servo::attach` explicitly return -1 if it fails
- Check for -1 in `Servo::move` because `servoIndex` might be 0
- Make `attach` / `detach` calls conditional on `SERVO_LEVELING`
- Move `SERVO_LEVELING` define to `Conditionals.h`

to make clear - it's not a question but an order.

and cleaned out some tabs.

Everything else is a rebased PR#2017 as proposed by @CptanPanic

by replacing the sequences (attach, write, delay, detach), and their conditions with the new function in Marlin.main.cpp
and removing the old configuration in the ABL part of Configuration.h

Sign flipped with #2167.
See also comments
https://github.com/MarlinFirmware/Marlin/issues/2040#issuecomment-118568355
and following.

- Also fix a bug in `M600` where `Z_MAX_ENDSTOP_INVERTING` is being
used as the index into the `destination[AxisEnum]` array.

here the other two occurences of `Z_PROBE_OFFSET_FROM_EXTRUDER` are
replaced with `zprobe_zoffset`.

verified via 1.0.2 tag that compiles by multiple people,
credit for the fix goes to paulusjacobus for the initial find and
sniffle for the correction so M851 works.

n_samples is an uint8_t .
Got printouts like:
50 of 2  ...